Arsenic is a naturally occurring element whose average level is 1.8 mg/kg of the earth's crust. Biological activity, weathering and volcanic eruption (97% natural input of arsenic) together with anthropogenic inputs, especially mining activities, are responsible for the distribution of arsenic in the environment. The most prevalent inorganic forms of arsenic are arsenate and arsenite, and the predominant species present is determined by either the chemical (redox) and/or microbiological condition of the aqueous or soil environment. Arsenite is more mobile in soil and sediment environments than arsenate because of its neutral, uncharged molecular state (H3AsO3) in common soil/sediment pH ranges (pH 5-8). Arsenite has been shown to be more prevalent under reducing conditions in natural water systems as arsenous acid species (H3AsO3, H2AsO3−, HAsO3−2)5 whereas arsenate (H3AsO4, H2AsO4−, HAsO4−2, ASO4−3) is more stable in oxygenated waters. Arsenite has a strong affinity for sulfur and readily adsorbs or coprecipitates with metal sulfides.
Arsenic is toxic to plants, animals and humans. It accumulates in living tissues because of its high affinity for proteins, lipids and other cellular components. Arsenic is also a carcinogen and exposure can lead to cardiovascular, pulmonary, immunological, neurological and endocrine disorders in addition to skin, lung, bladder and kidney cancers. Drinking water remains one of the most significant routes of arsenic exposure to humans. The recognition of the extent of arsenic toxicity resulted in the reduction of the maximum contaminant level (MCL) of arsenic from 50 ppb to 10 ppb. The reduction in the MCL of the metalloid requires the development of more efficient technologies than currently exist for the treatment of arsenic in water.
Current methods used to remove arsenic from water for human consumption include adsorptive media, anion exchange, coagulation/filtration, and membrane separation. The adsorptive media method involves removing arsenic ions by adsorption sites on an adsorption media. Granular activated alumina and various types of iron oxide composite have been used as adsorptive media with some degree of success. Both activated alumina and iron oxides are highly selective towards arsenate. Selectivity for arsenate over arsenite stems from the fact that in the pH range of 4 to 10, arsenite is predominantly neutral while arsenate is negatively charged, and can hence easily adsorb onto positively changed adsorption sites. Removal of arsenite using these methods would require pre-oxidation to arsenate.
The anion exchange method involves exchanging arsenic ions from a solution onto a resin. This method is specific for arsenate because of its negative charge at pH 4 or higher. Arsenite is neutral at in the pH range of 4 to 10. The resins used are usually in chloride from and during the exchange, the chloride will be replaced by arsenate on the exchange sites. The coagulation/filtration method also includes the removal of iron from water. Here iron and arsenate are simultaneously removed in areas with high iron concentrations. The efficiency of the arsenic removal system can be enhanced by adding iron (ferric sulfate) to the water. The other coagulant that is also used is alum. Finally, the membrane separation (reverse osmosis) method is a pressure-driven process that removes both arsenate and arsenite from water for small water systems.
Most current arsenic removal methods, except the membrane filtration method, are selective for arsenate. Selectivity for arsenate is a result of the negative charge on the arsenate species at pH 4 or higher. In addition to being arsenate specific, these methods are expensive, cumbersome and complex. Much of these processes are inaccessible to the poor communities that need them.
A method of treating water to remove contaminants such as arsenic is desired that is inexpensive for world-wide use. The method should and simple to use by the masses. The method should have few steps and should be able to treat water for any purpose and remove other contaminants such as most toxic heavy metals and metalloids in water.